Group III-V compound semiconductors using an InP substrate have a bandgap energy corresponding to the near-infrared region, and thus many studies on group III-V compound semiconductors have been carried out for the purpose of communications, biological examinations, imaging at night etc. Among these studies, a study that has been particularly focused on is a study on how group III-V compound semiconductor crystal layers having smaller bandgap energy and a plurality of light-receiving element that are arranged on these crystal layers to constitute a light-receiving unit can be formed on an InP substrate with good crystal qualities.
For example, results of the experimental production of a light-receiving element in which a sensitivity region is shifted to the long-wavelength side by using In0.82Ga0.18As as an absorption layer have been published (Non-Patent Document 1). In In0.82Ga0.18As, the bandgap is narrowed (the lattice constant is increased) by decreasing the proportion of gallium (Ga) and increasing the proportion of indium (In) in group III. When the proportion of In is increased as described above, the lattice constant of InGaAs increases, and a problem of an increase in lattice mismatch with an InP substrate occurs. In the above light-receiving element, this problem is tried to be solved by providing 12 to 20 InAsP graded layers, in which a proportion (As/P) is increased stepwise toward the absorption layer, between the InP substrate and the high-In-proportion InGaAs absorption layer. The increase in the lattice mismatch causes an increase in the lattice defect density, and inevitably results in an increase in the dark current. By interposing the above-mentioned graded buffer layers, the dark current is decreased to 20 to 35 μA. However, this current value is a value three orders higher than the dark current of a photodiode for optical communications, the photodiode including an InGaAs absorption layer. In addition, epitaxially growing a plurality of graded layers is not easy and increases the production cost.
In addition, a quaternary group III-V compound semiconductor composed of GaInNAs in which nitrogen (N) is further added to the group V element in InGaAs has been proposed (Patent Document 1). This quaternary group III-V compound semiconductor uses a function of narrowing the bandgap due to the addition of nitrogen. However, it is technically very difficult to grow GaInNAs crystals. In particular, in order to have sensitivity up to a wavelength of 3 μm and to achieve a lattice match with an InP substrate, it is necessary to increase the amount of nitrogen to about 10% (atomic percent in group V elements). However, it is very difficult to obtain a good crystal quality in a nitrogen amount of about 10%. In addition, in order to realize high sensitivity of a light-receiving element, it is necessary to increase the thickness of the GaInNAs layer containing a high concentration of nitrogen to 2 μm or more. However, it is more difficult to grow the nitrogen-containing crystal layer having such a large thickness with a good crystal quality.
In addition, results of a fabrication of a photodiode has been reported in which the cutoff wavelength is controlled to be 2.39 μm by forming a pn-junction with a p-type or n-type epitaxial layer using a type-II quantum well structure composed of InGaAs—GaAsSb (Non-Patent Document 2). This document describes that strain compensation is necessary in order to further shift the cutoff wavelength to the long-wavelength side. Thus, a photodetector with a cutoff wavelength of 2 to 5 μm having a strain-compensation quantum well structure composed of InGaAs—GaAsSb is proposed in this document.
A plurality of light-receiving elements is two-dimensionally or one-dimensionally arranged and is used as an imaging device or the like. However, unless the respective light-receiving elements are reliably separated from each other, a dark current, crosstalk, and the like are generated, and sharp images cannot be obtained. Photodiodes include a pn-junction. In the above-described photodiode, this pn-junction is formed by epitaxially growing, on a p-type semiconductor layer or an n-type semiconductor layer, a semiconductor layer having a polarity opposite to that of the semiconductor layer disposed thereunder. In this case, in order to separate the pn-junction that is widely formed as a plane into pn-junctions of respective light-receiving element, trenches for separating into respective light-receiving elements are provided. Such a trench is called an element separation trench and is formed by forming a planer pn-junction, and then performing mesa etching. In the formation of the element separation trenches of photodiodes for the near-infrared region, the photodiodes including an InP substrate, wet etching can be stopped at boundaries of respective layers by using an etchant having selectivity with respect to InP and InGaAs (Patent Document 2).
In the above wet etching method, however, it is difficult to control the shapes of respective light-receiving elements to be separated from each other with a high accuracy. For example, a trapezoidal taper may be formed in the longitudinal cross section of respective light-receiving elements, irregularities (recesses and protrusions) may be formed on a side face of a laminate depending on the type of semiconductor layer, or etchants may not sequentially intrude between adjacent light-receiving elements, and consequently, the formation of the trenches may be stopped in a halfway position and complete trenches may not be formed. It is very difficult to eliminate such incompleteness of element separation trenches. On the other hand, when a dry etching method is employed, damage occurs during etching, and it is difficult to stably produce photodiodes having a low dark current. Consequently, the yield decreases, thereby increasing the production cost.
As for the formation of an arrangement structure, i.e., the formation of an array, of a plurality of the above-mentioned light-receiving elements, except for Non-Patent Document 1, the pn-junction of the light-receiving elements proposed in the above-cited documents is formed between a p-type epitaxial layer and an n-type epitaxial layer, and the one-dimensional or two-dimensional arrangement of the light-receiving elements is performed by forming element separation trenches. Accordingly, the above-described problems in the formation of the element separation trenches occur.    [Non-Patent Document 1] T. Murakami, H. Takahashi, M. Nakayama, Y. Miura, K. Takemoto, D. Hara, “InxGa1-xAs/InAsyP1-y detector for near infrared (1 to 2.6 μm)”, Conference Proceedings of Indium Phosphide and Related Materials 1995, May, Sapporo, pp. 528-531    [Non-Patent Document 2] R. Sidhu, “A Long-Wavelength Photodiode on InP Using Lattice-Matched GaInAs—GaAsSb Type-II Quantum Wells, IEEE Photonics Technology Letters, Vol. 17, No. 12 (2005), pp. 2715-2717    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 9-219563    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-144278